Old Style Shower Drain
I have a building constructed in mid '40s. Shower drain on 3rd floor is draining slow for the past 6 months. Bathroom sink and toilet flow normally. There is no easily accessible means for shower drain clean-out. The drain assembly is apparently not conventional "P" trap or "S" trap that could be snaked. Water begins backing up after 2 -3 minutes after shower water is turned on. Standing water will drain after 2-3 minutes. Makes me think the problem may be farther down line. I have tried plunger, snaking from the roof through vent stacks, Drano, enzymes, forced hot water, etc. All these steps get limited or no improvement in flow. All other drains on 2nd and 1st floors run normally. I do hear a gurgle at 1st floor sink when 3rd floor toilet is flushed which makes me believe the problem may be in the vent.